This invention relates to a positioning device, also called a stage, for placing an object in a precise position, or for moving an object with precision. Devices of this kind have myriad applications and are of particular use in the manufacturing of semiconductors. Semiconductor manufacturing processes that involve the use of stages can include: lithography, inspection, pattern generation, wire bonding and others. In recent years, newer forms of many of these processes are performed in vacuum and require an environment with very low stray magnetic fields. This invention is especially suitable to these applications.
With traditional stages, in order to move an object in three or more degrees of freedom, it was necessary to employ three or more individual bearing systems, each with a single degree of freedom, X, Y and θz for instance, where θz is rotation about the Z axis. Each of these degrees of freedom had its own single degree of freedom drive such as a lead screw or linear motor. These single degree of freedom bearing and drive systems were cascaded in series with each other. This required a number of massive structural components in series with a number of bearings with their associated compliances. This resulted in modes of resonance at lower frequencies than desirable as well as friction, backlash, and other inaccuracies of movement.
More recently, in order to overcome these difficulties, stage devices have been devised with fewer components articulating with respect to each other. Devices of this kind include U.S. Patent Documents:
Re 27,289February 1972SawyerRe 27,436July 1972Sawyer4,485,339November 1984Trost4,506,204March 1985Galburt4,506,205March 1985Trost, Galburt4,507,597March 1985Trost4,655,594April 1987Wittekoek, Bouwer5,327,060July 1994Van Engelen, Bouwer
These devices are generally aimed at higher resonant frequencies, allowing higher bandwidth servo control, as well as minimization of friction through the use of fluid film bearings such as air bearings. Many of them specifically employ the use of mirrors attached to moving elements of the stage. The position of these mirrors may be measured precisely by interferometer systems that are used to provide feedback for positioning the stage.
Since the mirror is the element of the stage that is measured in the feedback system, ideally, the mirror should be connected as directly as possible to the drive elements in those degrees of freedom in which the mirror moves. Likewise, the mirror should ideally be connected as directly as possible to the bearings in those degrees of freedom in which the stage is constrained, by the bearings, not to move. This invention relates to a stage that accomplishes these goals in a more effective manner than in previous efforts.
The present invention relies on the use of gas bearings to essentially eliminate friction and its deleterious effects on accuracy and other aspects of stage performance such as vibration. Similarly, it makes use of Lorentz force drives, and specifically allows for the use of non-commutated Lorentz force drives. As is well known to those skilled in the art, these properties contribute significantly to the performance and accuracy of the stage.
In recent years, as the semiconductor industry has progressed toward smaller sizes of transistors and other features on electronic microcircuits, it has been moving gradually away from stages that operate in air to stages that operate in vacuum for lithography, pattern generation and inspection applications. This is because when light is used for these applications, it must be of shorter wavelength, and as the wavelength gets shorter, air becomes less transparent. Wavelengths that do not propagate well through air are known as “vacuum ultraviolet” and “extreme ultraviolet.” In addition, electron optical and other charged particle optical devices are used which also require vacuum. These charged particle optical systems are sensitive to stray magnetic fields that can be generated by a stage system, and particularly a Lorentz force system.
Previous stages have made use of gas bearings and Lorentz force motors in vacuum:
 4,417,770November 1983Tucker5,784,925July 1998TrostTrost, “Using Air Bearings in Vacuum to Control StageVibration,” Semiconductor International, July, 2002, pp 165-1686,445,440September 2002Bisschop et al
U.S. Pat. Nos. 4,417,770 and 5,784,925 employ flexible bellows to separate the exhaust of the gas bearings from the high vacuum environment. U.S. Pat. Nos. 5,784,925 and 6,445,440 as well as Trost, “Using Air Bearings in Vacuum . . . ” employ differentially pumped scavenging systems to limit the amount of bearing exhaust gas that flows into vacuum environments. U.S. Pat. No. 6,445,440 uses flat air bearings at a feed through between the atmosphere and the vacuum environment, and the Trost article describes bearings of a cylindrical (journal) construction entirely within the vacuum chamber.
Gas bearings are constructed to have two elements that move with respect to each other. Generally one element is fixed, and the other moves. The two elements have a pair of complementary conforming surfaces with a small gap between them that allows relative motion while maintaining a relatively fixed gap. These surface pairs, often referred to as articulating surfaces, may be constructed in a variety of shapes that allow different types of relative motion. Spherical bearing surface pairs, for instance, allow the moving part of the bearing to articulate in three rotational degrees of freedom. Cylindrical bearing surface pairs (journal bearings) allow motion in two degrees of freedom; a translation along the cylinder axis and a rotation about the cylinder axis. Conical bearing surface pairs allow a single degree of freedom defined as rotation about the axis of the cone. Flat surface pairs allow for relative motion in three degrees of freedom defined by a pair of orthogonal translations X and Y and a rotation about the mutually orthogonal Z axis. This rotation is referred to as θz. Bearing types are often defined by the shape of their articulating surfaces. Thus bearings with flat articulating surfaces are referred to as flat bearings.
The use of flat bearings is desirable since they are more easily constructed to the necessary tight tolerances and less expensive than other types. Similarly, the use of air bearings entirely within the vacuum chamber is desirable since the vacuum chamber tends to distort under the load of atmospheric pressure when the air is pumped out of the chamber. This distortion makes the use of air bearings at a feed through difficult. This invention employs flat bearings entirely within the vacuum chamber, not located at a feed through, to achieve maximum benefit from air bearing technology in vacuum.
Existing technology for vacuum stage systems employs fixed magnetic shielding attached to the vacuum chamber or comprising the vacuum chamber. This keeps magnetic fields from outside the chamber from penetrating to the interior. It also helps to reduce magnetic fields generated inside the chamber from exceeding requirements at critical areas close to the vacuum chamber wall or fixed shield. Generally magnetic components of the stage are placed outside the vacuum chamber, or within the vacuum chamber but at sufficient distance from critical areas to minimize stay fields. This forces current stage designers to limit the use of magnetic materials in the stage, or to choose magnetic components with very low stray fields, and to place them far from critical areas. The inventive device overcomes many of these limitations.